Low drop-out voltage regulators (LDO) are widely used in portable electronic equipment such as a cellular phone, digital cameras, and portable media players, etc., to provide a constant voltage power supply for analog and/or digital circuits. The power supply rejection ratio (PSRR) of the LDO, indicating the capability of suppressing power supply noise from its output, is normally of importance.
A conventional existing LDO is shown in FIG. 1. Such a circuit is e.g. disclosed in U.S. Pat. No. 7,205,831 B2. The existing LDO includes an error amplifier 10, power transistor 20, feedback resistors 30 and 31, load resistors 33 and 34, load capacitor 40. FIG. 1 also shows a power-supply 55, reference voltage Vref, as well as output voltage Vout 51. In the conventional existing LDO, as shown in FIG. 1, the power supply noise is suppressed by a negative feedback circuit comprising the error amplifier 10 and the power transistor 20.
FIG. 2 is a circuit diagram of a small signal equivalent of a voltage regulator circuit of FIG. 1. As shown in FIG. 2, a transconductor 21 and a resistor 36 represent the transconductance and the drain source resistance, respectively, of the power transistor 20 of FIG. 1. The PSRR for the LDO based on the small signal equivalent shown in FIG. 2 is given by:
                              P          ⁢                                          ⁢          S          ⁢                                          ⁢          R          ⁢                                          ⁢          R                =                                            A              v                                      A              dd                                =                                                    V                in                                            V                out                                      ≈                                          A                EA                            ⁢              β              ⁢                                                          ⁢                              R                op                            ⁢                              G                mp                                                                      
Av is the open loop gain of negative feedback loop of the LDO, Vin is the voltage of power-supply input 50 of the LDO, and Vout is the voltage of output 51 of the LDO.
Furthermore, AEA is the differential gain of the error amplifier 10, β a feedback factor which is the ratio of the resistance of resistor 30 to the sum of the resistances of the resistor 30 and 31, Rop is the drain source resistance of the power transistor 20, and Gmp is the transconductance of the power transistor 20.
There is a need for a low drop-out voltage regulator that, for example, consumes a relatively small area, and/or can provide an improved PSRR without necessarily increasing its open loop gain.